Equipment for processing semiconductor wafers in a plasma gas environment typically couple radio frequency (RF) power from the plasma gas to the wafer to effect surface treatment of the wafer (e.g., etching, deposition, etc). For instance, U.S. Pat. No. 4,617,079 discloses a parallel plate arrangement wherein a wafer is supported on a lower electrode, RF power from a low frequency generator passes through a low frequency network, RF power from a high frequency generator is combined with the low frequency RF power in a high frequency matching and combining network, and the combined signals are applied across upper and lower electrodes. In this arrangement, the high frequency matching and combining circuit can include a high frequency trap (capacitor and inductor in parallel) tuned to the frequency of the high frequency source for preventing signals generated by the high frequency source from being fed back to the low frequency source but allowing signals generated by the low frequency source to pass therethrough without being attenuated.
U.S. Pat. No. 4,948,458 discloses a parallel plate arrangement wherein the upper electrode is in the form of an electrically conductive coil located outside the plasma reaction chamber and by inducing an RF current in the coil a magnetic field is produced in a planar region parallel to the plane of the coil. The coil is driven by an RF generator which supplies power to a matching circuit having a primary coil and a secondary loop. A variable capacitor in series with the secondary loop adjusts the circuit resonant frequency with the frequency output of the RF generator and impedance matching maximizes efficiency of power transfer to the planar coil. An additional capacitor in the primary circuit cancels part of the inductive reactance of the coil in the circuit.
When processing semiconductor wafers in plasma gas environments, it is desired to uniformly process the entire surface of the wafer. For example, U.S. Pat. No. 4,615,755 discloses a plasma etching technique wherein uniformity of the wafer temperature is achieved by He backcooling of a wafer supported on a bowed electrode. By bowing the wafer away from the lower electrode with the cooling helium, cooling performance of the wafer is sacrificed in order to achieve etch uniformity. However, variations in the thickness of the wafer results in substandard control of the wafer bowing and thereby reduces the etch uniformity.
In plasma etching processes which use electrostatic (ESC) wafer clamping systems, the wafer cannot be bowed away from the surface of the electrode to control etch rate uniformity. Accordingly, other techniques are necessary for controlling the uniformity of the wafer surface treatment in plasma processing which are applied to ESC wafer clamping systems.